Ferroelectric Random Access Memory (FeRAM) is a type of high speed and non-volatile memory which utilizes the hyteresis properties of ferroelectric materials. While offering the same high-speed read and write capabilities as Dynamic Random Access memory (DRAM), FeRAM has clear advantages including non-volatility and low power consumption. For this reason, FeRAM has the potential to replace existing memory types such as EEPROM and SRAM, and development is proceeding in this area.
FIG. 1 shows the cross-sectional structure of a conventional ferroelectric memory cell. This structure is formed by the addition between the CMOS process and the wiring process of a ferroelectric process by way of a number of masks. Ideally, a memory-embedded LSI is realized by simply adding the memory section, without altering the processes used for the CMOS or wiring sections in any way.
However, from a practical standpoint, the introduction of a ferroelectric member requires the use of new practices and materials. For example, ferroelectric materials are extremely vulnerable to a deterioration of characteristics in a reducing atmosphere, and for this reason, the introduction of a hydrogen (H2) diffusion barrier film 118 made of TaOx or the like, or a passivation film 114 made of SiN or the like which prevents hydrogen diffusion, is required. The SiN film is formed by a sputtering method. If such a film is formed by plasma CVD (chemical vapor deposition), a reducing atmosphere would must be present, and the resulting film will contain a large amount of hydrogen. Furthermore, in some cases a Al2O3 film may be used instead of a SiN film.
To manufacture a ferroelectric capacitor, heat treatment at a high temperature in an oxidizing atmosphere is required to restore the characteristics of the ferroelectric material after sintering or etching. Consequently, the lower electrode or upper electrode must be made of a metal which has high oxidation resistance, a metal which remains conductive after oxidation, or a conductive oxide. One material which satisfies this requirement is platinum (Pt), which, despite being seldom used in existing LSI products, is a stable element that does not oxidize easily.
The conventional ferroelectric memory cell shown in FIG. 1 is manufactured by using a CVD method to deposit an insulating layer on a semiconductor substrate 101 on which transistors (102a, 102b, 102c) have already been formed by a known method. A BPSG film is then flowed to planarize the surface. Then, an insulating layer made of spin-on glass (SOG) is applied by spin coating and etched back to form a first insulating layer 103.
Next, photolithography and etching are performed to form a plurality of holes in the areas where the contact holes are to be located. Then, implantation of a contact material is performed, followed by heat treatment. After heat treatment, a metal film (Ti film) is deposited using a CVD method. After the Ti film is deposited, nitriding treatment is carried out in the same chamber. Next, a W film is deposited using a CVD method. Reference numeral 104 indicates a metal plug.
Next, a second insulating layer 105 is deposited by a sputtering method to serve as an adhesion layer for a lower electrode of a capacity insulating film. Then, the lower electrode 106 of the capacity insulating film, the capacity insulating film 107, and the upper electrode 108 of the capacity insulating film are deposited sequentially. The lower electrode 106 and the upper electrode 108 are formed from Pt using a sputtering method.
Next, the upper electrode 108 made of Pt is processed by photolithography and etching. Then, the capacity insulating film 107 made of SBT and the lower electrode 106 made of Pt are processed by photolithography and etching. After etching, heat treatment is performed with an object of restoring the ferroelectric properties. Then, a third insulating film 110 is deposited using a CVD method.
Next, contact holes are formed in the upper electrode 108 and the lower electrode 106 of the capacitor 109. After the contact holes are formed, heat treatment is performed with an object of restoring the ferroelectric properties. Subsequently, contact holes 112 are formed above the metal plugs 104. Then a barrier film 120 made of titanium nitride (TiN) is deposited.
Next, a main conductive film 113 made of an Al alloy is formed, and a conductive film 114 made of TiN which serves as an anti-reflection coating is formed thereon. Finally, the barrier film 120, the main conductive film 113, and the conductive film 114 are processed by photolithography and etching, thereby forming the wiring pattern. Reference numerals 117, 118, and 119 indicate secondary wiring, a hydrogen diffusion barrier film, and a passivation film, respectively.
The following are examples of documents disclosing the related art.
[Patent Document 1] Japanese Unexamined Patent Publication No. H08-181212
[Patent Document 2] Japanese Unexamined Patent Publication No. H06-151815
Pt, which is the electrode material, is highly reactive to the aluminum (Al) alloy typically used as the wiring material. After the Al alloy wiring is formed, heat treatment is performed under an O2 atmosphere at 400° C. for 30 minutes, for example, to restore the ferroelectric capacitor characteristics. At this time, the Al diffuses through the TiN crystal grains serving as the barrier metal, and a reaction occurs between the Al and Pt. This results in the formation of an Al2Pt compound which causes volume expansion of the contacting section, in some cases to the extent that the capacitor is ruined. This situation is shown in FIG. 2.
TiN has a columnar crystal structure, wherein the Al atoms diffuse through the space between TiN grains. Thus, one conceivable method for improving the barrier properties of the TiN film is to increase the thickness of the TiN film, thereby lengthening the diffusion path of the Al. However, increasing the thickness of the TiN film, which is formed using a sputtering method, promotes the formation of a TiN overhang at the top of the contact. This reduces the embeddability of the Al alloy film at the contacts with the ferroelectric electrode and at the other contacts, causing deterioration in the electrical characteristics. Furthermore, there is a concern that if insufficient quantities of the Al alloy are embedded in the contacts, variation in the shape of the contacts and other factors can cause variability in electrical characteristics between lots or within a given wafer.